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The Circadian Rhythm

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Describe and evaluate research on circadian rhythms, with reference to endogenous pacemakers and exogenous zeitgebers (9+16 marks) Circadian rhythms are those rhythms that last about 24 hours. The two best-known circadian rhythms are the sleep wake cycle and the body temperature cycle. People may think the reason they go to sleep and wake up at fairly regular times is because of changes in daylight - you feel sleepy when it gets dark and are roused by streaming sunlight; or perhaps a person's sleep wake cycle is governed by knowing what time of day it is. These are external cues from the environment called exogenous zeitgebers. Psychologists have investigated what happens when a person is free of such exogenous zeitgebers (external cues). One of the most memorable studies was conducted by the French cave explorer, Michel Siffre who is a specialist in the study of the human internal clock. He spent long periods of time living underground in order to study his own biological rhythms. Underground, in a cave, he had no external cues to guide his rhythms - no daylight, no clocks, no radio. He simply woke, ate and slept when he felt like it. ...read more.


Folkard et al (1985) conducted an experiment to see if external cues could be used to override the internal clock. A group of 12 people lived in a cave for three weeks, isolated from natural light and other time cues. These volunteers agreed to go to bed at 11.45 pm and to get up when it indicated 7.45 am. Initially the clock ran normally, but gradually they quickened the clock until it was indicating the passing of 24 hours when actually only 22 hours had passed. At the beginning, the volunteers circadian rhythms matched the clock, but, as it quickened, their rhythm no longer matched the clock and continued to follow a 24-hour cycle rather than the 22-hour cycle imposed by the experiment (except for one participant who did adapt to the 22-hour cycle). Overall, this suggests that the circadian rhythm can only be guided to a limited extent by external cues. However, there are exceptions as individual differences need to be taken into account. One important type of individual difference is the cycle length: research has found that circadian cycles in different people can vary from 13 to 65 hours (Czeisler et al, 1999). ...read more.


looked at the learning ability of 12-13 year old children who had stories read to them at either 9am or 3pm. After one week, the afternoon group (higher core body temperature) showed both superior recall and comprehension, retaining about 8% more meaningful material. This suggests that long-term recall is best when body temperature is highest. There is evidence that temperature changes do actually cause the changes in cognitive performance. Giesbrecht et al (1993) lowered body temperature (by placing participants in cold water) and found that cognitive performance was worse on some tasks. However, other research has found that the link is spurious. For example, Hord and Thompson (1983) tested cognitive performance in a field rather than lab situation and didn't find any correlation between core temperature and cognitive performance. It may be that the higher core body temperature leads to increase physiological arousal and this leads to improved cognitive performance (Wright et al, 2002) Circadian rhythms are part of the daily lives of humans. They cue our levels of alertness, our need for sleep, and our time of waking. Research shows that light, hormones, food, and a variety of other factors are important in determining circadian rhythms. Our fairly consistent sleep pattern suggests they are innate and not learnt meaning there is an internal or endogenous mechanism - the biological clock. However, this can be overridden by psychological factors, such as anxiety. ...read more.

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